Like any bitter divorce, Sen. Arlen Specter’s departure from the Republican Party was followed immediately by bitter recriminations and the central question of who betrayed whom.

Specter said the Republican Party had “moved right” and left him. Republican leaders cast him as an opportunist and “disrespectful” besides.

So which is it? Man of principle or rank opportunist?

Few know better than Sen. Richard Shelby of Alabama.

“A man does what he needs to do, I guess,” Shelby told POLITICO on Tuesday. The Alabama senator became a Republican one day after the GOP takeover of Congress in 1994.

The particulars of Specter’s switch must sound familiar to Shelby, who found himself holding a Senate seat in a state where hostility toward his national party posed a threat to his future. And like Specter, who has been reproached by national party leaders for his wayward voting practices, Shelby was rebuked by the Clinton administration for his own departures from party dogma.

Party switching, of course, is a grand tradition in American politics, one that follows broad shifts in political power and geography.

See also

According to the Senate historian, 20 other senators have switched parties since 1890. And while the switchers virtually always cast their moves as matters of principle in renouncing a party that has abandoned them, each case also includes a calculus of self-preservation and ambition. Few give up safe seats or join a party out of power.

George Norris, for instance, left the Republican Party to run — with President Franklin D. Roosevelt’s backing — as an independent in 1936, but most switches of the second half of the 20th century ran in the other direction. South Carolina Democratic Sen. Strom Thurmond left his party in 1964, perhaps the most prominent of a generation of Southern politicians who abandoned the Democratic Party for the GOP, first on issues of race and segregation and later on questions of defense and taxation.

“Philosophically, I was not tuned in to the Democratic Party; I had not been. The party had become more liberal,” Shelby said in 1994.

That’s what they always say.

“Since my election in 1980, as part of the Reagan big tent, the Republican Party has moved far to the right,” Specter said Tuesday.

But the ascendancy of the conservative movement is decade-old news, and while large forces may press politicians in one direction or the other, the exact timing of party switches is hardly coincidental.

Phil Gramm of Texas, then a member of the House, switched parties in 1983 after Democratic leadership removed him from his House Budget Committee seat over his support of President Ronald Reagan’s economic package. Gramm then resigned his seat and won it back in a special election. A year later, he ran for the Senate as a Republican and won.

In the wake of the Republican takeover of 1994, two senators — Shelby and Colorado’s Ben Nighthorse Campbell — became Republicans. Vermont Sen. Jim Jeffords left the Republican Party in 2001, turning years of discomfort inside his party into a moment of leverage and celebrity, as cars around the Northeast sprouted “Thank You, Jim” bumper stickers.

Jeffords’ move was an early sign of the latest trend: The Northeast has become a mirror image of the South, as Republicans have systematically lost congressional seats across the region. Not a single New England district is represented by a Republican, and New York, New Jersey and Pennsylvania are becoming increasingly hostile country.

“The South is the other side of the coin,” said Alan Abramowitz, a professor of political science at Emory College in Atlanta. “You had the conservative Democrats switching to the Republican Party, and now you’ve got the moderate Republicans switching to the Democrats in the Northeast.”

The timing of Specter’s move, though, had less to do with broad historical trends than with a coming primary.

“The party left [Specter] a while ago — but he didn’t see the light until his career was threatened,” Abramowitz said.

Specter’s move is particularly dramatic, by historical standards, because previous Republicans have not jumped with both feet into the opposing party, but rather departed their party for independent status. That’s what Jeffords did, and it’s where the idiosyncratic New Hampshire Sen. Bob Smith landed during a brief defection from the GOP.

“The individual matters. In the case of Jeffords, he just couldn’t bring himself to become a Democrat,” said Ross Baker, a political scientist at Rutgers University. “He was too deeply encoded with the Republican DNA.

“Specter simply doesn’t have that long-term connection to the Republican Party,” he said. “He is a Republican by opportunism and then a Democrat by opportunism.”

Specter’s departure also brings two species of Republicans that much closer to extinction: He is the last Jewish Republican in the Senate and one of the last of the liberal big-city Republicans, both lines that trace their pedigree to Sen. Jacob Javits of New York.

Now, observers’ attention is turning to whether the geographic and ideological trends will pull any other Republicans out of their party. The two Maine senators criticized their own party’s conservatives Tuesday but signaled that they wouldn’t be departing anytime soon. Sen. Olympia Snowe cited her “ethnic heritage, Spartan side” in continuing to fight.

“I will never switch parties,” Sen. Susan Collins said. “It’s not good for our party, and even more importantly, it’s bad for the country.”